1. Field of the Invention
This invention relates to a method of and an apparatus for controlling the air fuel ratio of an internal combustion engine.
2. Description of the Prior Art
An exhaust gas purifying system is conventionally known wherein catalytic converter rhodium for purifying exhaust gas of an internal combustion engine is disposed in an exhaust system of the internal combustion engine to purify exhaust gas of the engine.
It is already known that the exhaust gas purifying efficiency of such an exhaust gas purifying system can be improved by oscillating the air fuel ratio proximate the theoretical air fuel ratio (stoichiometric mixture ratio).
To this end, an oxygen concentration sensor of the .lambda. type (which denotes an oxygen concentration sensor which presents a sudden change in output value therefore proximate a predetermined air fuel ratio (theoretical air fuel ratio, and such sensor will be hereinafter referred to as O.sub.2 sensor) is conventionally provided in an intake manifold on the upstream side of a catalytic converter. Taking notice of the fact that the output of such O.sub.2 sensor presents a change from an on-state to an off-state, that is, a change from a high voltage level to a low voltage level or vice versa at or proximate the theoretical air fuel ratio, the output of the O.sub.2 sensor is fed back to control the air fuel ratio so that the air fuel ratio may have a value proximate the theoretical air fuel ratio. Such control is called O.sub.2 feedback control.
In such O.sub.2 feedback control, an output of the O.sub.2 sensor is compared with an on/off criterion voltage (reference value), and if, for example, the O.sub.2 sensor output is higher than the criterion voltage, the air fuel ratio is controlled so that it may become lean, but on the contrary, if the O.sub.2 sensor output is lower than the criterion voltage, the air fuel ratio is controlled so that it may be rich.
With such conventional O.sub.2 feedback control, however, there is the possibility that, if the O.sub.2 sensor used for the feedback control suffers from a secular change or deterioration, the reliability in control may be deteriorated. Further, where the dispersion of O.sub.2 sensors is great, the dispersion of emission levels is also great. Also this may possibly give rise to deterioration in reliability in control.
Further, since the maximum frequency of variations in air fuel ratio is restricted by a delay (wasteful time) in conveyance of gas from a fuel supply station to a location of the O.sub.2 sensor as well as a delay in responding of the sensor, there is the possibility that the capacity of the catalyzer may not be exhibited sufficiently.
Means has thus been proposed for further improving the exhaust gas purifying characteristic of an exhaust gas purifying system of an internal combustion engine. Such means is disclosed, for example, in Japanese Patent Laid-Open No. 56-118535 wherein the air fuel ratio of air fuel mixture introduced into catalytic converter rhodium is varied positively so that a high purifying efficiency of the catalytic converter rhodium may be attained over a wide range of the air fuel ratio.
With the conventional means, however, since the median of variations of the air fuel ratio is invariable, there still is the possibility that the air fuel ratio may be varied proximate the maximum purifying efficiency of the catalytic converter rhodium due to a difference among air fuel ratio controlling apparatus or a secular change of the air fuel ratio controlling apparatus.